1. Field of the Invention
The present invention relates generally to severing tubes, such as guide thimbles from a fuel assembly top nozzle and, more particularly, is concerned with a positive drive mechanism for uniform feed of an internal tube cutter in order to circumferentially cut the guide thimble from the inside out without generating chips.
2. Description of the Prior Art,
In most nuclear reactors, the reactor core is comprised of a large number of elongated fuel assemblies. Conventional designs of these fuel assemblies include a multiplicity of fuel rods held in an organized array by grids spaced along the fuel assembly length. The grids are attached to a plurality of hollow tubular guide thimbles. Top and bottom nozzles on opposite ends of the fuel assembly are secured to the guide thimbles which extend above and below the opposite ends of the fuel rods. Typically, at the top end of the fuel assembly, the guide thimbles are attached in passageways provided in the adapter plate of the top nozzle.
During operation of such fuel assembly in a nuclear reactor, a few of the fuel rods may occasionally develop cracks along their lengths resulting primarily from internal stresses, thus establishing the possibility that fission products having radioactive characteristics may seep or otherwise pass into the primary coolant of the reactor. In order to remove and replace these defective fuel rods in reconstitution of the fuel assembly, the top nozzle of the fuel assembly must first be removed. One approach to removal of the top nozzle is to sever each guide thimble from the top nozzle adapter plate by circumferentially cutting the guide thimble from the inside out just below its region of attachment to the adapter plate. Precision internal tube cutters devised for this purpose are illustrated and described in U.S. Pat. Nos. 4,589,309 and 4,621,549 to Nokovich, which patents are assigned to the assignee of the present invention.
The guide thimbles are typically composed of stainless steel. Thus, a controlled positive rate of feed of the cutting wheel (or wheels) of the precision internal cutter is believed essential to ensure satisfactory cutting of the stainless steel without producing chips and to prolong the life of the carbide cutting wheels. A variety of mechanisms have been developed over the years for feeding internal tube cutters and other tools. Representative of such prior art mechanisms are the ones disclosed in U.S. Pat. Nos. to Thieme (512,631 and 531,662), Richard (558,422), Miller (647,104), Chamberlain et al (819,516), Ashley (949,136), Renner (1,095,229), Wiedeke (1,162,893), Baranoff (1,478,692), Wiedeke (1,756,717 and 1,789,572), Armstrong (1,883,453), Hoorn et al (2,295,789), Armstrong (2,446,734), Reck (2,508,539), Fisher (2,700,818), Venghiattis (3,396,795), Alexander et al (3,583,822) and Idzik et al (4,455,746).
However, none of these prior art mechanisms seem to be particularly well-suited for driving a cutter at a controlled positive rate of feed to sever guide thimbles from the top nozzle of an irradiated fuel assembly in a manner which does not produce chips and debris. Consequently, a need exists for development of such an internal tube cutter drive mechanism.